Baselining techniques in force-based touch panel systems

ABSTRACT

In connection with establishing a touch location on a touch screen, a number of reference levels are developed. One or more of the reference levels may be used to determine a differential change in the touch signal between a zero touch force condition and a touch event. In one approach, one or more reference levels are selected to compensate for various conditions affecting the touch screen at the time a touch is applied. Using one or more reference levels to compensate for conditions affecting the touch signal at the time touch location information is obtained can provide improved accuracy in determining touch location.

FIELD OF THE INVENTION

[0001] The present invention is directed generally to a touch sensor,and more particularly to a method and system for accurate determinationof a touch location on a touch screen.

BACKGROUND

[0002] A touch screen offers a simple, intuitive interface to a computeror other data processing device. Rather than using a keyboard to type indata, a user can transfer information through a touch screen by touchingan icon or by writing or drawing on a screen. Touch screens are used ina variety of information processing applications. Transparent touchscreens are particularly useful for applications such as cellphones,personal data assistants (PDAs), and handheld or laptop computers.

[0003] Various methods have been used to determine touch location,including capacitive, resistive, acoustic and infrared techniques. Touchlocation may also be determined by sensing the force of the touchthrough force sensors coupled to a touch surface. Touch screens thatoperate by sensing touch force have several advantages over othertechnologies mentioned above. First, force sensors do not require thetouch surface to be composed of special materials that may inhibitoptical transmission through the touch surface, as in a resistive touchsensor. Further, force sensors do not rely on a lossy electricalconnection to ground, as required by a capacitive touch screen, and canbe operated by a finger touch, gloved hand, fingernail or othernonconductive touch instrument. Unlike surface acoustic wave technology,force sensors are relatively immune to accumulations of dirt, dust orliquids on the touch surface. Finally, a force sensor is less likely todetect a close encounter as an actual touch, which is a common problemwith infrared touch screens.

[0004] Forces detected by touch screen force sensors reflect a varietyof static and dynamic factors in addition to the touch force. Thesefactors may be considered noise sources with respect to the touchsignal. Noise may be introduced through the touch screen electronics, orit may be mechanical in nature. Electrical noise may be introduced, forexample, in the sensor, amplifier, data conversion or signal processingstages. Mechanical noise may arise from torsion of the touch screen,movement of the touch screen device, vibration of the touch screen, andother transient factors. In addition, the touch screen force sensors maybe affected by the weight of the touch surface and preloading forcesapplied to the force sensors during manufacture.

[0005] The touch force changes throughout the duration of a touch. Atouch in a single location produces a touch force signal that increasesas the touch is applied and then decreases as the touch is removed. Thetouch may also be moved across the surface of the touch screen,generating a dynamic touch signal at each force sensor. Accuratedetermination of touch presence and location requires analysis of forcesignals generated by the touch force, as well as elimination of thesteady state and transient noise signals from various ancillary factorsaffecting the touch screen at a particular time.

SUMMARY OF THE INVENTION

[0006] In general terms, the present invention relates to a method andsystem for detecting the location of a touch on a touch sensor. Thepresent invention is particularly useful when combined with amicroprocessor-based system operating a display device enhanced by atransparent touch screen.

[0007] In accordance with one embodiment of the present invention, amethod for determining a touch location of a touch on a touch screeninvolves acquiring a plurality of reference levels for a forceresponsive touch signal, selecting one or more of the plurality ofreference levels based on information acquired from the touch signal,and determining touch location using the selected reference levels.

[0008] Another embodiment of the present invention includes developing afirst and a second reference level for a force responsive touch signaland determining the touch location using at least one of the developedreference levels.

[0009] In accordance with another embodiment of the invention, a methodfor establishing a reference level for a touch signal includes sensing aquiescent touch signal prior to an application of a touch force,detecting a touch signal responsive to the application of the touchforce, and establishing a reference level for the touch signal based ona value of the touch signal acquired contemporaneously with thedetection of the touch force.

[0010] Another embodiment of the invention, a touch screen systemincludes a touch surface and a plurality of touch sensors physicallycoupled to the touch surface. Each of the touch sensors produces asensor signal in response to a touch applied to the touch surface. Acontrol system, coupled to the touch sensors, receives sensor signalsand receives sensor signals, develops a plurality of reference levelsfor a force responsive touch signal, selects one or more of theplurality of reference levels based on information acquired from thetouch signal, and determines the touch location using the selectedreference levels.

[0011] Another embodiment of the invention is directed to a touch screendisplay system. In this embodiment, a touch screen display systemincludes a touch surface and a plurality of touch sensors physicallycoupled to the touch surface. Each of the touch sensors produces asensor signal in response to a touch force applied to the touch surface.A control system, coupled to the touch sensors, receives sensor signalsand receives sensor signals, develops a plurality of reference levelsfor a force responsive touch signal, selects one or more of theplurality of reference levels based on information acquired from thetouch signal, and determines the touch location using the selectedreference levels. The touch screen display system further includes adisplay for displaying information through the touch screen.

[0012] Another embodiment of the invention is directed to a displaysystem including a touch screen system, a display for displayinginformation, and a processor coupled to the touch screen and the displayfor processing data displayed on the display and information receivedfrom the touch screen control system. The touch screen system includes atouch surface and a plurality of touch sensors physically coupled to thetouch surface. Each of the touch sensors produces a sensor signal inresponse to a touch force applied to the touch surface. A controlsystem, coupled to the touch sensors, receives sensor signals andreceives sensor signals, develops a plurality of reference levels for aforce responsive touch signal, selects one or more of the plurality ofreference levels based on information acquired from the touch signal,and determines the touch location using the selected reference levels.The touch screen display system further includes a display fordisplaying information through the touch screen.

[0013] In accordance with a further embodiment of the invention, asystem includes means for developing a plurality of reference levels fora force responsive touch signal and means for determining a location ofa touch on a touch screen using one or more of the plurality ofreference levels.

[0014] Yet another embodiment of the invention involves means fordeveloping a first and a second reference level for a force responsivetouch signal and means for determining touch location using at least oneof the first or the second reference levels.

[0015] In accordance with another embodiment of the invention, a systemfor establishing a reference level for a touch signal includes means forsensing a quiescent touch signal prior to an application of a touchforce, mans for detecting a touch signal responsive to the applicationof the touch force, and means for establishing a reference level basedon a value of the touch signal acquired contemporaneously with thedetection of the touch force.

[0016] In accordance with another embodiment of the invention, acomputer-readable medium configured with executable instructions forcausing one or more computers to perform a method of determining alocation of a touch on a touch screen, the method including acquiring aplurality of reference levels for a force responsive touch signal,selecting one or more of the plurality of reference levels based oninformation acquired from the touch signal, and determining touchlocation using the selected reference levels.

[0017] The above summary of the present invention is not intended todescribe each illustrated embodiment or every implementation of thepresent invention. The figures and the detailed description which followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention may be more completely understood in considerationof the following detailed description of various embodiments of theinvention in connection with the accompanying drawings, in which:

[0019]FIG. 1 schematically illustrates a top view of a touch screen withforce sensors located at the corners of the touch screen in accordancewith an embodiment of the invention;

[0020]FIG. 2 schematically illustrates a cross-section view of acapacitive force sensor in accordance with an embodiment of theinvention;

[0021]FIG. 3 schematically illustrates a perspective view of a touchscreen with force sensors located at the corners of the touch screen inaccordance with an embodiment of the invention;

[0022]FIG. 4 is a block diagram of a touch screen and touch screencontroller in accordance with an embodiment of the invention;

[0023]FIG. 5 illustrates a threshold point and location point inaccordance with an embodiment of the invention;

[0024]FIG. 6 is a flowchart illustrating a method of acquiring abaseline reference value in accordance with an embodiment of theinvention;

[0025]FIG. 7 illustrates a fast rise trigger for detectingloss-of-quiescence in accordance with an embodiment of the invention;

[0026]FIG. 8 illustrates a slow rise trigger for detectingloss-of-quiescence in accordance with an embodiment of the invention;

[0027] FIGS. 9A-9C are conceptual flowcharts illustrating variousmethods of touch location processing in accordance with the invention;

[0028]FIG. 10 is a flowchart illustrating a method of determining touchlocation for a streaming touch using a baseline reference value and abackground reference value in accordance with an embodiment of theinvention;

[0029]FIG. 11 is a block diagram of a data processing system using atouch sensing interface in accordance with an embodiment of theinvention; and

[0030]FIG. 12 illustrates a touch screen controller in accordance withan embodiment of the invention.

[0031] The invention is amenable to various modifications andalternative forms. Specific embodiments have been shown by way ofexample in the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0032] In the following description of the illustrated embodiments,references are made to the accompanying drawings that form a parthereof, and in which is shown by way of illustration, variousembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized, and structural andfunctional changes may be made without departing from the scope of thepresent invention.

[0033] As stated above, and for other reasons stated below which willbecome apparent upon reading the present specification, there is a needfor a method and a system for rapid and accurate detection of thepresence of a finger touch or an instrument touch on a touch surface toimprove the determination of touch location for various touch types.There exists a further need for such a method and system that accuratelydetects the presence and location of a touch as the touch is movedacross the touch pad.

[0034] The present invention is applicable to touch sensing techniquesand is believed to be particularly useful when features of the inventionare combined with a data processing system operating a display deviceenhanced by a transparent touch screen. For example, the touch screen ofthe present invention may be used in a desktop, handheld or laptopcomputer system, a point-of-sale terminal, personal data assistant(PDA), or a cell phone. Although described in combination with amicroprocessor-based system, the touch screen device of the presentinvention may be combined with any logic-based system; if desired.

[0035] The present invention is directed to determining one or moretouch signal reference values to improve the accuracy of touch locationdetermination. A touch may be sensed by a number of touch sensors thatproduce force responsive sensor signals. A touch signal may be derivedfrom a single sensor signal or by combining sensor signals from two ormore force sensors. Accurate touch location determination involvesmeasuring an increase in magnitude of one or more touch signals above atouch force reference amplitude, considered to represent a zero touchforce condition.

[0036] Determination of the touch location involves analysis of touchsignals produced by the touch screen sensors. Prior to the applicationof a touch, the touch signal remains at a quiescent level. A touch in asingle location characteristically produces a touch signal thatincreases in magnitude when the touch is applied and then decreases inmagnitude as the touch is removed. The rapid change in the touch signalindicates a touch presence. A touch may be a continuing touch whereinthe touch remains on the touch surface for a period of time. Forexample, the touch may be present in a single location for a period oftime. Further, the touch may be a “streaming touch,” wherein the touchis applied at one location, moved across the surface of the touchscreen, and removed at another location, causing the generation of acontinuously changing signal at each sensor.

[0037] The touch signal may be affected by a variety of transient andsteady-state noise components that prevent touch signal from remainingat a constant zero level during a quiescent period when no touch ispresent. Transient noise factors may include noise introduced in thetouch screen electronics or by mechanical vibration, torsion or othermotion of the touch screen. Steady-state noise factors may include, forexample, preloading of the touch sensors or weight of the touch screen.

[0038] Noise components of the touch signal produce errors in touchlocation calculation. Errors affecting the accurate determination of thetouch location may be categorized into three groups. First, error mayarise from touch-independent noise. Touch-independent error arises fromnoise sources or disturbances not correlated with the touch itself, andcan fluctuate unpredictably. Second, there may be static error in thereported touch location. Static touch location error is a reproduciblefunction of the touch location and also of the steady-state force.Finally, dynamic touch location error may be introduced by the touchitself. Dynamic touch location error may occur during or immediatelyfollowing a rapid change in the touch force.

[0039] Detection of a touch presence and location depends upondiscerning a change in the touch signal caused by a touch force appliedto the touch screen. Detection of a touch presence and determination oftouch location depends upon measuring a differential change in the touchsignal with respect to an established reference level representing azero touch force condition. A touch signal may generally be affected byone or more errors, such as those described above, resulting in anon-zero touch signal for a zero touch force condition. The touch signalmay be altered by long term effects, such as low frequency noise andlong term drift. The touch signal may also be altered by short termeffects constituting transient conditions, such as an operator squeezingor shaking the touch screen device.

[0040] Accurate touch location determination in accordance with thepresent invention, depends upon acquiring one or more touch signalreference levels representing a zero touch force condition. Thereference levels are used to determine a differential change in thetouch signal between a zero touch force condition and a touch event.According to the methods of the present invention, one or more touchsignal reference levels representing a zero force condition are acquiredand retained. The one or more touch signal reference levels represent azero touch force reference level for purposes of the touch locationcalculation and may be selected to compensate for various conditionsaffecting the touch screen. By way of example, a first touch signalreference level may be used to compensate for low frequency noise andlong term drift of the touch signal. A second touch signal referencelevel may be used to compensate for short term effects that may bepresent at the instant a touch event begins. Either the first or thesecond reference level, or both, may be selected for use in the touchlocation calculation based on the type or touch signal detected orexpected. Selection of one or more reference levels to compensate forconditions affecting the touch signal at the time the touch locationmeasurement is made can provide improved accuracy in determining touchlocation.

[0041] A generalized diagram of a touch screen is illustrated in FIG. 1.A touch surface 100 is arranged proximate to one or more touch forcesensors. In the embodiment shown, the touch sensors 110, 120, 130, 140are arranged at four corners of a rectangular touch surface. Althoughthe touch screen illustrated in FIG. 1 is rectangular with sensorslocated at the corners, various configurations using three or more touchsensors with differing touch surface shapes may also be used.

[0042] The sensors, 110, 120, 130, 140, may be, for example, smallcapacitive force sensors constructed of two capacitor plates separatedby a gap. A capacitive force sensor may be arranged so that when a touchforce of sufficient magnitude and direction is applied to the touchsurface, one capacitor plate deflects towards the second plate. Thedeflection alters the distance between the capacitor plates, changingthe capacitance of the sensor. The touch force may be measured bycontroller circuitry as a change in an alternating electrical signalapplied to the touch sensor. One embodiment of a capacitive force sensorappropriate for use in touch screen applications is described in U.S.Patent Application, U.S. Ser. No. 09/835,040, filed Apr. 13, 2001 andentitled “Method and Apparatus for Force-Based Touch Input,” which ishereby incorporated by reference in its entirety. In this particularembodiment, the sensor measures the applied force based on the change ofcapacitance of a capacitive element.

[0043] A touch surface 210, or overlay, is located within a structure orhousing 215. The touch surface 210 is typically transparent to allowviewing of a display or other object through the touch surface. In otherapplications, the touch surface 210 can be opaque.

[0044] The structure or housing 215 may be provided with a large centralaperture through which the display may be viewed. If desired, theundersurface of the housing 215 may be seated directly against thesurface of such a display, over the border surrounding its active area.In another embodiment, as mentioned above, the overlay may be replacedby a structure including a display unit, such as an LCD.

[0045] A capacitive sensor 220 may be positioned between the touchsurface 210 and the housing 215. An interconnect 225, with attachmentlands 233, may be coupled to the housing 215 by soldering, cementing, orby other methods. A conductive area forms a first conductive element 234on the interconnect 225. A second conductive element 235 with a centralprotrusion 240, for example a dimple, may be attached to the lands 233of the interconnect 225 by soldering, for example. A small gap 280 isformed between the first conductive element 234 and the secondconductive element 235, either by the shape of the second conductiveelement 235, or by the process of attaching the second conductiveelement 235 to the interconnect 225. The width of the gap 280 may beapproximately 1 mil, for example. A capacitor is formed by theconductive elements 234, 235 separated by the gap 280.

[0046] An optional bearing surface 270 may be interposed between thetouch surface 210 and the second conductive element 235. This mayprotect the touch surface 210 from indentation or from damage by theprotrusion 240, especially in cases where the overlay is made of softermaterial. The bearing surface 270 may also mount to the touch surface210 through a thin layer (not shown) of elastomer or of highly pliableadhesive, thereby providing a lateral softening function. It will beappreciated that, in normal operation, the touch surface 210 or bearingsurface 270 is in contact with the protrusion 240: these elements areshown separated only for clarity in the illustration.

[0047] The second conductive element 235 combines the functions of aspring and a capacitor plate. As a perpendicular force is applied to thetouch surface 210, the second conductive element 235 flexes, decreasingthe width of the gap 280 and increasing the capacitance of the sensor220. This change in capacitance may be measured and related to the forceapplied to the touch surface 210. Although a touch screen usingcapacitive force sensors is described, other types of force sensors maybe used in a similar manner, including, for example, piezoelectricsensors and strain gauge sensors.

[0048] One of the advantages of a force-based touch screen is that thenumber of optically distinct layers positioned between the display unitand the user is low. Typically, the overlay positioned over the displayunit is a single layer of glass or relatively stiff polymer, for examplepolycarbonate or the like, which may be chosen for suitable opticalqualities. This contrasts with other types of touch screen, such asresistive or capacitive touch screens, that require several, potentiallyoptically lossy, layers over the display unit. The electricallyconductive thin films required in resistive or capacitive touch screenstypically have a high index of refraction, leading to increasedreflective losses at the interface. This is a particular problem inresistive screens where there are additional solid/air interfaces andwhere antireflection coatings are not useful, since the conductivelayers must be able to make physical contact. A screen overlay for aforce-based touch screen, however, has only its upper and lowersurfaces; these may be treated to reduce reflective losses and to reduceglare. For example, the overlay may be provided with matte surfaces toreduce specular reflection, and/or may be provided with anti-reflectioncoatings to reduce reflective losses.

[0049] A perspective view of a touch screen is schematically illustratedin FIG. 3. A touch surface 300 is shown disposed proximate to forcesensors 310, 320, 330, 340 located at the corners of the touch surface300. As a stylus, finger or other touching device 352 presses the touchsurface 300, a touch force 355 is exerted upon the touch surface 300 atthe touch location 350. The touch force 355 creates forces F1, F2, F3,and F4 on the force sensors 310, 320, 330, 340 perpendicular to thetouch surface 300. The force sensors 310, 320, 330, 340 may be drivenwith an alternating electrical signal. The perpendicular forces F1, F2,F3, and F4 cause a change in the capacitance of the force sensors 310,320, 330, 340, thereby causing the signal coupled through the forcesensors 310, 320, 330, 340 to change. The force responsive signalsderived from the force sensors 310, 320, 330, 340 may be used tocalculate touch location information.

[0050] In the exemplary embodiment illustrated in FIG. 4, a touchsurface 405 is configured proximate to four force sensors 401, 402, 403,404 arranged at the corners of the touch surface 405. The sensors 401,402, 403, 404 may be chosen from a variety of sensing technologies,including capacitive, piezoelectric and strain gauge sensors. Thesensors 401, 402, 403, 404 measure the force of a touch detected at thesensor locations and are coupled to drive/sense circuitry 410, 420, 430,440 located within the controller 450. Alternatively, some components ofthe drive/sense circuitry may be located near the corresponding sensor.An energizing signal developed in the drive circuitry for each sensor412, 422, 432, 442 is used to energize the sensors 401, 402, 403, 404.Each sensor 401, 402, 403, 404 produces a touch force signalcorresponding to a touch force applied to the sensor through the touchsurface 405. The touch force signal developed by each sensor 401, 402,403, 404 is detected by sense circuitry 411, 421, 431, 441 locatedwithin the controller 450.

[0051] Analog voltages representing the touch force at each sensorlocation are produced by the sense circuitry 411, 421, 431, 441. Thesevoltages may then be sampled, and the held values multiplexed insampling circuitry 460. The sampling circuitry 460 provides for samplingthe analog force sensor signals at a rate sufficient to produce arepresentation of the signals sufficient for touch locationdetermination. The sampled signals are directed to an analog to digital(A/D) converter 470 where the signals are digitized. The digitized touchsignal samples are directed to processor circuitry 480 for furthersignal processing, such as filtering 482, and for calculations todetermine a touch location. The processor circuitry 480 may be coupledto memory circuitry 486 for storage of, for example, data representingthe sampled touch signal, as well as various touch screen calibrationparameters. The processor circuitry 480 may perform a number ofadditional controller functions, including controlling the systemtiming, the multiplexer circuitry 460 and the A/D converter 470.

[0052] It may be found advantageous to implement the touch screencontrol system 450, or its equivalent, on a single mixed-mode integratedcircuit chip. In such an implementation, it may be advantageous toreplace sampling circuitry 460 and A/D converter 470 with a set ofdelta-sigma converters operating in parallel, one for each sensorchannel.

[0053] One method for timing the touch location calculation is describedin commonly owned U.S. patent application entitled “Method for ImprovingPositioned Accuracy for a Determined Touch Input,” identified underDocket Number 57470US002 which is hereby incorporated herein byreference in its entirety. According to this method, touch location maybe calculated from data gathered at a preferred time within the touchsignal time profile. Accurate determination of the preferred time fortouch location calculation by this method may entail two decisions: 1) adecision that a touch event has begun, and 2) a decision that apreferred time for making a touch location measurement has occurred. Oneexample of this method is illustrated in FIG. 5. Prior to theapplication of a touch on the touch screen, the touch signal remains ata quiescent level 510 during the quiescent period 515. When a touch isapplied, the touch signal rises in response to the force exerted on thescreen. As the signal rises in response to a touch, a point in the touchsignal is reached 520 ending the quiescent period 515. The transitionbetween the quiescent period 515 and the end of the quiescence 525 maybe determined by various techniques. The beginning of a touch event maybe determined when the touch signal exceeds a predetermined active touchthreshold 530 at active threshold point 535. The active touch signal 540progresses through an active touch period 545 by rising above the activesignal threshold 530 and then declining as the touch is removed. Alocation point 550 may be established at a preferred time for making ameasurement to determine the touch location. The preferred time forobtaining touch location information may be based upon the shape of thetouch signal.

[0054] A touch signal may represent a combination of force responsivetouch sensor signals. The sensor signals are sampled at a ratesufficient to capture an adequate representation of the signals for thepurpose of determining a touch presence and location. For example, thesensor signals may be sampled at a rate of 183 Hz, although othersampling rates may be used. The signals are digitized by an analog todigital (A/D) converter and various digital signal processing steps maybe performed, including scaling, filtering, and signal correctionaccording to previously determined calibration factors. One or morereference levels representing a zero touch force condition may beacquired from touch signal samples taken during a quiescent period whenthe touch screen is not loaded by a touch force. The reference levelsused to calculate the touch location depend on the type of touch signaldetected.

[0055] A reference level in the present context represents the currentbest estimate of the correct zero-force reference level for the touchsignal data stream. A reference level that compensates for low frequencynoise and long term drift reflects the quiescent touch signal over arelatively long period of time. Such a reference level, denoted hereinas a baseline reference, may be acquired, for example, by performing areal-time, moving, weighted average of the quiescent touch signalsamples.

[0056] Another technique for acquiring the baseline reference is byadjusting the baseline reference in the direction of the touch signalsample value as each sample is taken so long as the sample value isclose to the level of the current baseline. When the sample value isclose to the value of the current baseline reference, such as withinplus or minus a margin having a value equal to 20% of the active-touchthreshold, the current baseline reference may be changed by a fixed butsmall increment. The small increment of change is chosen as a value thattracks the effects of drift on the quiescent signal but prevents largechanges in the baseline caused by brief signal fluctuations. Forinstance, the increment may be chosen such that when applied uniformlyin the same direction at every sample time, the result is to slew thebaseline at 2% of the active-touch threshold per second. When the samplevalues are not close to the current baseline estimates, the currentbaseline level remains unchanged.

[0057] A tentative baseline may also be formed from the ongoing data. Ifthe sample values stay sufficiently close to the tentative baseline fora period of time, the tentative baseline may be substituted as thecurrent baseline level. The procedure of using a tentative baseline toreplace a current baseline may be necessary to provide an escape from acondition where erroneous baseline values may cause false touchdetection. Some normal occurrences, such as changing the tilt angle of atouch screen, can cause a large step change in the zero-touch forcesignal. The technique of acquiring a tentative baseline that can besubstituted for the current baseline within an appropriate interval oftime limits the effect of an erroneous baseline level. Further, anytouch force, even when carefully sustained, shows considerablefluctuation in force level. Thus, the appearance of steady zero-touchforce values far from the presumed current baseline indicates that theforce values are in error and the current baseline should be replacedwith a correct baseline.

[0058]FIG. 6 is a flowchart illustrating a technique of acquiring acurrent baseline level in accordance with an embodiment of theinvention. The next available set of digitized sample values areacquired 605. Various signal processing steps may be applied to thedigitized touch signal including scaling, decimation, filtering andcalibration adjustment 615. If the absolute value of the differencebetween the sample value and the current baseline is less than apredetermined value 620, e.g. 20% of the active-touch threshold, thetentative baseline counter is reset 625 and the current baseline isadjusted with the sample value 630. If the sample value is not close tothe current baseline 620, but is close to the tentative baseline 632,then the tentative baseline is updated with the sample value 635 and thetentative baseline counter is incremented 640. If the tentative baselinecounter reaches its timeout value 645, then the current baseline isreplaced with the tentative baseline 650. If the tentative baselinecounter has not reached its timeout value, the current baseline remainsunchanged 655.

[0059] Calculation of the touch location may be performed, for example,using combinations of the force responsive touch sensor signals. Theforce responsive signals generated by the touch sensors may be used tocalculate various touch signals, including the moment about the y-axis,M_(y), moment about the x-axis, M_(x), and the total z-direction force,F_(Tz). The coordinates of the touch location are determined from theforce sensor signals as in Equation 1, assuming a reference point in thecenter of the touch screen, ideal conditions, with no errors, backgroundfluctuations or disturbances present other than the touch force.$\begin{matrix}{{X = \frac{M_{y}}{F_{Tz}}}{Y = \frac{M_{x}}{F_{Tz}}}} & \lbrack 1\rbrack\end{matrix}$

[0060] where

M _(y)=(F 2+F 4)−(F 1+F 3)−M _(y) _(—) _(baseline);

M _(x)=(F 1+F 2)−(F 3+F 4)−M _(x) _(—) _(baseline); and

F _(Tz) =F 1+F 2+F 3+F 4−F _(Tz) _(—) _(baseline)

[0061] According to Equation 1, M_(y), M_(x), and F^(Tz) represent thedifferential values between the corresponding touch signals and theirassociated baseline reference values.

[0062] In addition to acquiring and updating a current baseline valuerepresenting the long term zero touch force reference level, a fasterfluctuating background reference level may also be acquired. Thebackground level compensates for short term effects on the touch screen,such as an operator squeezing or twisting the device during a touch.Such a background reference may represent the touch signal state at amoment in time just prior to the application of touch force.

[0063] The background reference responsive to short term effects on thetouch screen may be established using one or more touch signal valuesacquired contemporaneously with the detection of the touch force. Theone or more touch signal values used to establish the reference aretouch signal values acquired within an interval close in time to thedetection of the touch force. In one example, the touch signal valuesused to establish the reference are acquired within 100 ms of thedetection of the touch force. Touch location may then be computed fromthe differential value formed by subtracting the background referencefrom the touch signal at the location point.

[0064] The baseline reference may be subtracted from both the backgroundreference and the touch signal at the location point. The differentialvalue between the touch signal at the location point and the backgroundreference is the same, regardless of whether or not a slow baselinevalue has previously been subtracted from both the background referenceand the touch signal. Thus in an alternative embodiment of theinvention, a background reference may be formed and subtracted in thetouch location computation without first subtracting the baselinereference. In this situation, the background reference compensates forall speeds of fluctuation in the zero touch force reference level. Sucha simplified approach may be particularly suitable in button typeapplications that do not require continuing response to a continuingtouch.

[0065] In one embodiment of the invention, both a baseline referencelevel and a background reference level are acquired in accordance withone of the methods set forth above. Two touch locations are calculatedafter a touch presence is detected. The coordinates of the touch downlocation are calculated as set forth in Equation 1 above. Thiscalculation produces touch location data referenced to the baselinereference value only. A second touch location may be calculated thatcorrects the values produced by Equation 1 for background fluctuations.The coordinates of the second touch location may be calculated as inEquation 2. $\begin{matrix}{{{X\_}_{bc} = \frac{M_{y} - M_{y\_ bc}}{F_{Tz} - F_{Tz\_ bc}}}{{Y\_}_{bc} = \frac{M_{x} - M_{x\_ bc}}{F_{Tz} - F_{Tz\_ bc}}}} & \lbrack 2\rbrack\end{matrix}$

[0066] where M_(y), M_(x), and F_(Tz) represent the corresponding touchsignals with their associated baseline reference values subtracted, andM_(y) _(—) _(bc), M_(x) _(—) _(bc), and F_(Tz) _(—) _(bc) represent thebackground reference values associated with the M_(y), M_(x), and F_(Tz)touch signals, respectively.

[0067] A system employing both a baseline reference and backgroundreference may provide greater accuracy during continuing touches usedfor drag-and-drop operations, and better detection of very slowlyapplied touches. Further the use of both a baseline reference and abackground reference may be particularly advantageous when a very slowlyapplied touch is also a continuing touch.

[0068] The touch signal, F(t_(n)), may be taken to represent a set ofscalar values describing the touch signal state at time t_(n). This setmay, for instance, comprise the raw readings from the sensors, or maycomprise a sufficient set of linear combinations of these, such as totalforce and moments about the X and Y axes. The set may also comprisefiltered signal values, and may reflect the subtraction of a slowbaseline, if used. Many different combinations are possible within thescope of the present invention. For concreteness and simplicity in thediscussion below, however, a complete touch signal sample value may betaken to comprise the combination of total force with X-axis and Y-axismoments, with the terms “touch signal level” and “touch signalmagnitude” taken to denote the total force component alone.

[0069] According to one method of the present invention, the backgroundreference level corresponds to a delayed sample of the touch signal. Forexample, if the current touch signal sample, F(t_(n)), corresponds tothe touch signal at time t_(n), the background reference may be chosento correspond to a touch signal sample F(t_(n−m)) taken at time t_(n−m),m samples earlier than the current touch signal. In an alternate method,the background reference level may correspond to a low-pass filtering ofthe current touch signal F(t_(n)); or, to the value of a low passfiltering of the touch signal m samples prior to the current touchsignal sample.

[0070] In one embodiment, the background reference may be taken withrespect to the time of the location point. For instance, the locationpoint may be established by determining the presence of a peak in thesignal level at time t_(n), and then the location may be computed fromthe components of the touch signal at the location point, represented byF(t_(n)), less the components of the touch signal at time t_(n−m),represented by F(t_(n−m)). In this simple procedure, the value of m mustbe large enough to allow for the full evolution of the touch profile toits location point. For instance, at a sampling rate of 200 Hz, m may beset to a value between 10 and 20 to provide a 50 to 100 ms delay.

[0071] With m large, however, the delay will often be larger thannecessary, thus allowing the true zero touch force level to fluctuatemore than necessary between t_(n−m) and t_(n). On the other hand,smaller values of m may occasionally incorporate some of the touchitself into the background reference. Incorporation of some of the touchsignal rise into the background reference may not generally affect thelocation computed for the initial touch location at the time when thetouch is first applied to the touch screen, denoted the touch downpoint.

[0072] Allowing incorporation of the touch signal rise into thebackground reference may create problems with continuing touches usedfor a moving touch. Any time a background reference reflects some of thetouch force applied to one location, but is used to compute a touchlocation when the touch has moved elsewhere, there is potential forerror. This problem may also affect a rapid sequence of touches, unlessthe method of acquiring the background is adapted to require that thesignal level return to a quiescent level, such that the backgroundreference is not affected by the trailing edge of a preceding touch.

[0073] In another embodiment, the difficulties associated withcontamination of a background reference with a touch signal rise may beminimized by acquiring the background reference with respect to atrigger event early in the touch profile. For example, the backgroundreference may be established at a time when the touch signal has risenrapidly or to a magnitude signaling the end of the quiescent period. Thetriggering event signaling loss-of-quiescence may comprise the earlierof either: (i) a fast rise trigger, marked by a rise rate of the signalexceeding a predetermined minimum rate, such as 1% of the active-touchthreshold per millisecond, or (ii) a slow rise trigger, marked by thesignal level rising beyond a loss-of quiescence threshold represented bya predetermined value above the slow baseline, such as 20% of theactive-touch threshold. Since such a loss-of-quiescence threshold may bechosen to be well below an active-touch threshold, a loss-of-quiescenceevent will always occur before a location point is achieved.

[0074] A background reference acquired at the triggeringloss-of-quiescence event may be held unchanged while used in subsequenttouch location calculations, and continue to be held until the touch isremoved and the quiescent condition reestablished. If loss of quiescenceis recognized at time t_(n), then the background reference may beF(t_(n−m)). Due to the close proximity to the beginning of the touchforce profile, this may now correspond to a delay of, for instance, 20mS, with m set to 4 at a sampling rate of 200 Hz. The delay between theloss-of-quiescence event and the background reference is reduced incomparison with the situation in which the background reference is takenwith respect to the touch location point, as previously discussed.

[0075]FIGS. 7 and 8 show the transition between a quiescent signalperiod and an active signal period marked by a loss of quiescence eventdetermined by a fast rise trigger and a slow rise trigger, respectively.In FIG. 7, a fast rise trigger event is illustrated. The touch signaltransitions rapidly from a quiescent signal 710 to an active signal 720.A loss of quiescence 702 marks the transition from the quiescent period701 representing a period of zero touch force to a state which may ormay not develop into an active signal period 703 representing a periodin which a touch has been determined to have been applied to the touchscreen. As depicted here, a typical touch of adequate magnitude has beenapplied to the touch screen, and a full sequence of processing events isdepicted in response. Loss of quiescence 702 is determined the firsttime that the touch signal slope exceeds a predetermined value 730. Thesignal slope is determined by subtracting the signal magnitude at sampletime t_(n−1) from the signal magnitude at the current sample time t_(n).If this difference exceeds a value 735, corresponding to slope 730 overthe interval represented by the difference, then a fast-rise event hasoccurred, ending the quiescent period. When the quiescent period ends,the nature of the trigger event, which is a fast rise trigger, may berecorded for use in later processing.

[0076] A working set of background reference values may be continuallyupdated from a delayed value of the touch signal, F(t_(n−m)), during thequiescent period 701. Equivalently, the background reference may betaken just at the end of the quiescent period 702 at t_(n). In eithercase, the background reference is left unchanged after the loss ofquiescence 702. The fast-rise trigger 735 ends the quiescent period 701in the illustrated case, since it occurs prior to the signal levelexceeding a loss-of-quiescence threshold 760 at point 765.

[0077] Once the signal level rises above active-touch threshold 750 atactive-touch threshold point 755, a location point may be sought, and aninitial touch location, denoted a touch down location, may be reported.Prior to the signal level dropping below a falling-touch threshold 751at point 756, additional location points may be reported, depending inpart upon the duration of the touch. After the signal level drops belowthe falling touch threshold 751, a final touch location, denoted thetouch up location, may report the last position of touch prior to theloss of the active-touch condition. The falling touch threshold 751 maybe set equal to active touch threshold 750. Alternatively, the fallingtouch threshold 751 may be set to a somewhat smaller value, such as 72%of active touch threshold 750, to minimize unexpected dropouts duringwhat the user intends to be continuing touches.

[0078] After the signal level falls below an allowed quiescencethreshold 761 at point 766, which may have the same value asloss-of-quiescence threshold 760, additional appropriate conditions mayagain indicate a quiescent period. A minimal additional condition maycomprise detection of at least one signal value below that of theimmediately preceding sample, to establish that the quiescence conditionis not erroneously being reasserted in the interval just after thefast-rise trigger clears it.

[0079]FIG. 8 illustrates a slow rise trigger. The processing method andparameters are unchanged from the example of FIG. 7, but in thissituation, the touch signal rises relatively slowly from a quiescenttouch signal 810 to an active signal 820. A loss of quiescence 802 marksthe transition from the quiescent period 801 representing a period ofzero touch force to an active signal period 803 representing a period inwhich a touch is applied to the touch screen. Loss of quiescence 802 isdetermined the first time that the touch signal exceeds a predeterminedvalue 760 at a point 865. The touch signal passes through theloss-of-quiescence threshold 760 at time t_(n) without slope threshold730 being first exceeded. A slow-rise event occurs when the touch signalexceeds the loss-of-quiescence threshold 760, ending a quiescent period.When the quiescent period ends, the nature of the trigger event, whichis a slow rise trigger, may be recorded for use in later processing. Inother respects, the touch is processed as in FIG. 7.

[0080] Once the signal level rises above active-touch threshold 750 atactive-touch threshold point 855, a location point may be sought, and atouch down location, may be reported. Prior to the signal level droppingbelow a falling-touch threshold 751 at point 856, additional locationpoints may be reported. After the signal level drops below the fallingtouch threshold 751, the touch up location may be reported as the lastposition of touch prior to the loss of the active-touch condition. Thefalling touch threshold 751 may be set equal to active touch threshold750. Alternatively, the falling touch threshold 751 may be set to asomewhat smaller value as previously described. After the signal levelfalls below a quiescence threshold 761 at point 866, which may have thesame value as loss-of-quiescence threshold 760, the touch signal mayagain enter a quiescent period if additional conditions are metindicating quiescence, as previously discussed in connection with FIG.7.

[0081] A number of variations on the embodiment just described may beconsidered. In one variation, the conditions for determining a return toa quiescent period may further include the requirement that a number ofsuccessive preceding sample values, such as three, span a narrowmagnitude range, such as 5% of the active-touch threshold value. This isparticularly applicable to rapidly applied touches with force profilesthat nearly or partly overlap. It may preserve background valuesreflective of a true zero-force level present at some earlier time,preventing them from being updated with a later set contaminated withforce of a touch that has not yet fully disappeared from the signal.

[0082] In another variation, developing and subtracting the baselinereference may be omitted. A signal level to be employed in thresholdcomparisons may then be taken as the difference between the currenttouch signal level, and the touch signal level reflected in a set ofbackground values that is continuously updated as long as the quiescencestate is set. In this situation, the baseline reference is notsubtracted from either the current touch signal or the backgroundreference. Such a variation may be best adapted to applications where acontinuing touch response is not required, nor a response to very slowlyapplied touches.

[0083] In another variation, improved accuracy may be achieved byextrapolating a background reference at the time of the location point.Suppose that methods as discussed previously have been used to acquire afirst set of background reference values, and that these represent ameasured quiescent point at time t_(o−p) some p sample times prior to alocation point at time t_(o). An additional delay may also be provided,such that a second set of background values may be acquired with thesecond set representing a measured quiescent point at time t_(o−p−q)some p+q sample times prior to the location point. A set of backgroundchange rates may now be computed by dividing the differences of thevalues for t_(o−p) minus the values for t_(o−p−q) by q. An extrapolatedset of background reference values for the location point may now beformed by adding the background change rates multiplied by p to thefirst set of background reference values, corresponding to sample timet_(o−p). Locations computed in this manner may be most accurate forapplications in which the fluctuation in zero-touch-force signals isdominated by medium speed events, which do not alter their rate ofchange greatly in the time it takes a touch to develop.

[0084] In another variation, improved accuracy may be achieved byinterpolating background values to the time to of the location point.Suppose that the methods discussed previously have been used to acquirea first set of background values, corresponding to time t_(o−p), andthat a second set is acquired after removal of the touch, correspondingto time t_(o+r), the first instant in which quiescence may be deemed tohave returned. A set of background change rates may now be computed bydividing the differences of the touch signal values for t_(o+r) minusthe touch signal values for t_(o−p) by the time period p+r. Aninterpolated set of background reference values for the location pointmay now be formed from the first set of background reference values,corresponding to t_(o−p), by adding the background change ratesmultiplied by p. Touch signal values from the location point may bestored, and the reporting of touch location delayed until theinterpolated background may be computed. Locations computed in thismanner may be most appropriate for applications in which the predominanttouch type is a tap touch, and in which background correction must be asaccurate as possible.

[0085] The methods of the present invention involving extrapolating orinterpolating reference values to a time the touch location informationis obtained are applicable to force-based touch screens. In addition,the methods of extrapolating or interpolating reference values havebroader applicability outside of force-based techniques in connectionwith a number of methods other used for touch sensing systems. Forexample, extrapolating or interpolating a reference level to the timetouch location information is obtained may be advantageously employed toincrease touch location accuracy in touch sensing systems usingcapacitive, resistive, acoustic or infrared techniques.

[0086] According to a method of the invention, in broad and generalterms, a touch location on a touch screen is calculated using one ormore particular reference levels representing the zero touch force levelfor a touch signal. Each reference level compensates for a particularcondition affecting the touch signal. The particular reference levelsused may be selected to compensate for various touch signal conditionsdetected or expected at a time a touch location measurement is made. Theselection of reference levels based upon touch signal conditions resultsin improved accuracy in the determination of touch location.

[0087]FIG. 9A is a flowchart conceptually illustrating a method of touchlocation processing in accordance with the present invention. A numberof reference levels for a touch signal are acquired 910 from thequiescent signal. One or more of the reference levels are selected asthe touch signal reference level based on information acquired from thetouch signal 920. The touch location is determined using one or more ofthe touch signal reference levels 930.

[0088] Another method of the invention is conceptually illustrated inFIG. 9B. A first reference level and a second reference level aredeveloped 940, 950. The touch location is determined using one or bothof the first and the second reference levels 960.

[0089] A method of establishing a background reference level isillustrated in the flowchart of FIG. 9C. A quiescent touch signal issensed prior to the application of a touch force 970. The application ofa touch force is detected 980. A background reference level isestablished based on a value of the touch signal acquiredcontemporaneously with the detection of the application of the touchforce 990.

[0090] The selection of the reference level used to determine touchlocation is based upon the type of the touch signal. In the case ofsingle-point touches, either quick tap touches or more deliberate slowtouches, use of the background reference produces accurate results. Thebackground reference compensates for short term effects that are presentfor the duration of a brief touch. However, in the case of a continuingtouch signal, calculations using the background level as the referencemay produce less accurate results.

[0091] A continuing touch generally occurs over a longer time periodthan the short term effects compensated for by the background level.Thus, the background level may become an increasingly inaccuratereference value over time. In addition, in the case of a slow touch,initiated by a slow rise trigger, the background level may becontaminated with some fraction of the active touch signal produced bythe force of the touch. To mitigate these inaccuracies, the touchlocation calculation may be processed so that the touch location fadessmoothly from a touch location calculated using the background level asthe signal reference, to the touch location calculated using only thebaseline reference as the signal reference. Thus a weight W may beassigned to the background-corrected location, and a location to beoutput computed as:

X _(—) _(out) =WX _(—) _(bc)+(1−W)X

Y _(—) _(out) =WY _(—) _(bc)+(1−W)Y  [3]

[0092] The fading may be based upon the distance by which the currentstreaming touch point is separated on the screen from the touch downpoint. In this situation, W may be associated with a distance traveledby the streaming touch. The effect of the background level may becompletely removed once the movement of the touch has reached apredetermined amount, for example 20% of the touch screen width. Valuesof X and Y retained from the touch down point may be denoted X_(td) andY_(td). For convenience, the movement distance D may be taken to be thegreater of the absolute values of X-X_(td) and Y-Y_(td). Then if S isthe screen size, we may set: $\begin{matrix}{W = {{Max}\left\lbrack {{1 - \frac{D}{0.2S}},0} \right\rbrack}} & \lbrack 4\rbrack\end{matrix}$

[0093] According to another embodiment, all streaming touches may beprocessed so that the background level is gradually removed from thetouch location calculation based upon the passage of time. Thus, W maydecline from 1 to zero linearly over a period of, for instance, onesecond.

[0094] Inaccuracies in the touch location calculation for streamingtouches are primarily associated with touch signals exhibiting a slowrise. The inaccuracy occurs because the slow touch may be contaminatedwith some portion of the active touch signal. Nevertheless, touchlocation accuracy may be increased by processing all streaming touchesin the above manner, regardless of whether the loss-of-quiescence eventwas triggered by a slow rise.

[0095] A flowchart illustrating a method for calculating touch locationaccording to the principles of the invention is illustrated in FIG. 10.A baseline level 1010 and a background level 1020 are acquired by themethods discussed above. The touch location is calculated using thebaseline level as in Equation 1. The touch location is calculated usingthe background level as in Equation 2. If the touch is not a streamingtouch 1030, the touch location is calculated using the backgroundreference level 1050. If the touch is a streaming touch 1030 the touchlocation is initially calculated using the background reference level.Use of the background reference is transitioned to the baselinereference level by one of the previously discussed methods 1045.

[0096] A touch screen of the present invention may be advantageouslyimplemented in various data processing systems. Turning now to FIG. 11,a block diagram of a data processing system 1100 using an integratedtouch screen and display is shown in accordance with an embodiment ofthe present invention. The system 1100 uses a transparent touch screen1106 arranged above a display 1108 suitable for data processingapplications, such as an LCD display. Other displays may be used, suchas a cathode ray tube (CRT) display, plasma display, light emittingdiode (LED) display, organic electroluminescent display, or the like.The display 1108 may require display controller circuitry 1109 forinterfacing the display with the data processor computer 1110. A touchscreen controller 1107 includes the drive/sense circuitry describedabove in addition to a touch screen controller processor according to anembodiment of the present invention.

[0097] The data processor 1110 may include various components dependingupon the computer system application. For example, the data processormay include a microprocessor 1112, various types of memory circuitry1114, a power supply 1118 and one or more input/output interfaces 1116.The input/output interfaces 1116 allow the data processing system toconnect to any number of peripheral 1/O devices 1120 such as keyboards1121, pointing devices 1122, and sound devices 1123, includingmicrophone and speakers. The data processing system may additionallyinclude a mass data storage device 1130, for example, a hard disk driveor CD ROM, and may be networked to other data processing systems througha physical or wireless network connection 1140.

[0098]FIG. 12 illustrates a touch screen system 1200 in accordance withthe present invention, wherein the processes illustrated with referenceto FIGS. 1-10 may be tangibly embodied in a computer-readable medium orcarrier, e.g. one or more of the fixed and/or removable data storagedevices 1210 illustrated in FIG. 12, or other data storage or datacommunications devices. One or more computer programs 1220 expressingthe processes embodied on the removable data storage devices 1210 may beloaded into various memory elements 1230 located within the touch screencontroller 1240 to configure the touch screen system 1200 for operationin accordance with the invention. The computer programs 1220 compriseinstructions which, when read and executed by the touch screen systemprocessor 1250 of FIG. 12, cause the touch screen system 1200 to performthe steps necessary to execute the steps or elements for detecting thelocation of a touch on a touch screen in accordance with the principlesof the present invention.

[0099] The present invention should not be considered limited to theparticular examples described above, but rather should be understood tocover all aspects of the invention as fairly set out in the attachedclaims. Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

What is claimed is:
 1. A method for determining a touch location of atouch on a touch screen, comprising: acquiring a plurality of referencelevels for a force responsive touch signal; selecting one or more of theplurality of reference levels based on information acquired from thetouch signal; and determining the touch location using the one or moreselected reference levels.
 2. The method of claim 1, further comprisingusing more than one reference level of the plurality of referencelevels.
 3. The method of claim 1, further comprising transitioning fromusing one of the plurality of reference levels as a touch signalreference to using another of the plurality of reference levels as theselected reference level during the touch.
 4. The method of claim 3,wherein transitioning from one reference level to another referencelevels occurs as the touch is moved across the touch screen.
 5. Themethod of claim 1, further comprising eliminating an effect associatedwith the one or more selected reference levels from the determined touchlocation after a predetermined time interval.
 6. A method fordetermining a touch location of a touch on a touch screen, comprising:developing a first reference level for a force responsive touch signal;developing a second reference level for the force responsive touchsignal; and determining the touch location using at least one of thefirst and second reference levels.
 7. The method of claim 6, whereinboth the first and second reference levels are used to determine thetouch location.
 8. The method of claim 6, wherein developing the firstreference level comprises performing a weighted average of two or moretouch signal samples taken during a touch signal quiescent period. 9.The method of claim 6, wherein developing the first reference levelcomprises adjusting the first reference level by a predetermined amountwhen a difference between a magnitude of a touch signal sample and amagnitude of the first reference level is less than a predeterminedvalue.
 10. The method of claim 9, wherein the first reference is notupdated during the time that the touch is applied to the touch screen.11. The method of claim 6, wherein developing the first reference levelcomprises: developing a tentative first reference level; and saving thetentative first reference level as the first reference level if thedifference between the touch signal and the tentative first referencelevel is less than a predetermined amount for a predetermined number oftouch signal samples.
 12. The method of claim 11, wherein developing thetentative first reference level comprises: acquiring two or more touchsignal samples during a quiescent period of the touch signal; andadjusting the tentative first reference level by a first predeterminedamount if a first difference between a touch signal sample and a firstreference value is greater than a second predetermined amount, and asecond difference between the touch signal sample and the tentativefirst reference level is less than a third predetermined amount.
 13. Themethod of claim 6, wherein developing the second reference levelcomprises using a touch signal sample taken at a predetermined timebefore touch location information is first obtained.
 14. The method ofclaim 13, wherein touch location information is first obtained at a peakof the touch signal.
 15. The method of claim 13, wherein thepredetermined time before touch location information is obtained is in arange of about 50 to 100 ms.
 16. The method of claim 6, whereindeveloping the second reference level comprises acquiring the secondreference level in response to a triggering event associated with anapplication of a touch.
 17. The method of claim 16, wherein the secondreference level is updated using one or more touch signal samples beforethe triggering event is detected.
 18. The method of claim 16, whereinthe second reference level is held unchanged during the time the touchsignal is not in a quiescent period.
 19. The method of claim 18, whereinthe touch signal is determined to be in a quiescent period when thetouch signal is below a predetermined threshold and at least one touchsignal sample has a value less than an immediately preceding sample. 20.The method of claim 18, wherein the touch signal is determined to be inthe quiescent period if a predetermined number of successive samplesremain within a predetermined range of values.
 21. The method of claim18, wherein the triggering event is a rise rate of the touch signalexceeding a predetermined rate.
 22. The method of claim 18, wherein thetriggering event is a magnitude of the touch signal exceeding apredetermined value.
 23. The method of claim 6, wherein developing thesecond reference level comprises using an extrapolated value as thesecond reference level.
 24. The method of claim 23, wherein using anextrapolated value as the second reference level comprises: acquiring afirst touch signal sample during a quiescent period; acquiring a secondtouch signal sample during the quiescent period, the second touch signalsample acquired a predetermined number of sample times after the firsttouch signal sample; determining a rate of change of the touch signalduring the quiescent period using the first and the second touch signalsamples; extrapolating a value of the touch signal at the time the touchlocation information is obtained from the first touch signal sample, thesecond touch signal sample and the rate of change of the touch signal;and using the extrapolated value as the second reference level.
 25. Themethod of claim 6, wherein developing the second reference levelcomprises using an interpolated value as the second reference level. 26.The method of claim 25, wherein using an interpolated value as thesecond reference level comprises: acquiring a first touch signal sampleduring a quiescent period; acquiring a second touch signal sample whenthe touch signal returns to a quiescent state following application of atouch; determining a rate of change of the touch signal between thefirst touch signal sample and the second touch signal sample;interpolating a value of the touch signal at the time the touch locationinformation is obtained from the first touch signal sample, the secondtouch signal sample and the rate of change of the touch signal; andusing the interpolated value as the second reference level.
 27. Themethod of claim 6, wherein determining the touch location includestransitioning from using the first reference level to using the secondreference level.
 28. The method of claim 27, wherein transitioning fromusing the first reference level to using the second reference levelcomprises: calculating touch signal location values using each of thefirst and second reference levels; assigning weights to each touchsignal location value; and determining the touch location based upon theweighted touch signal location values.
 29. The method of claim 28,wherein the weights assigned to each touch signal location value varywith distance as the touch is moved across the touch screen.
 30. Themethod of claim 27, wherein transitioning from using the first referencelevel to using the second reference level comprises eliminating aneffect associated with either the first or the second reference levelfrom the touch location determination when the touch is moved apredetermined distance.
 31. The method of claim 27, whereintransitioning from using the first reference level to using the secondreference level comprises eliminating an effect associated with eitherthe first or the second reference level from the touch locationdetermination after a predetermined time interval.
 32. The method ofclaim 27, wherein transitioning from using the first reference level tousing the second reference level comprises calculating X and Y touchlocation coordinates as: X=WX _(ref2)+(1−W)X _(ref1), Y=WY_(ref2)+(1−W)Y _(ref1), where: W is a weighting factor, X_(ref2) andY_(ref2) are touch location coordinates calculated using the secondreference level, and X_(ref1) and Y_(ref1) are touch locationcoordinates calculated using the first reference level.
 33. The methodof claim 32, wherein the weighting factor, W, is calculated as:$W = {{Max}\left\lbrack {{1 - \frac{D}{0.2S}},0} \right\rbrack}$

where D is a greatest distance traveled in either an X direction of a Ydirection, and S is a size of the touch screen.
 34. The method of claim32, wherein the weighting factor, W, declines linearly from 1 to 0 overa period of one second.
 35. A method of establishing a reference levelfor a touch signal, comprising: sensing a quiescent touch signal priorto an application of a touch force; detecting a touch signal responsiveto the application of the touch force; and establishing a referencelevel for the touch signal based on one or more values of the touchsignal acquired contemporaneously with the detection of the touch force.36. The method of claim 35, wherein establishing the reference level forthe touch signal based on one or more values of the touch signalacquired contemporaneously with the detection of the touch forcecomprises establishing the reference level using one or more values ofthe touch signal occurring within a predetermined time interval prior tothe detection of the touch force.
 37. The method of claim 36, whereinthe predetermined time interval is about 100 ms.
 38. The method of claim35, wherein establishing a reference level for the touch signal based ona value of the touch signal acquired contemporaneously with thedetection of the touch force comprises using as the reference level atouch signal sample taken a predetermined time before touch locationinformation is first obtained.
 39. The method of 38, wherein touchlocation information is first obtained at a peak of the touch signal.40. The method of 38, wherein the predetermined time before touchlocation information is obtained is in a range of about 50 to 100 ms.41. The method of 35, wherein detecting the touch signal responsive tothe application of the touch force comprises detecting a triggeringevent associated with the application of the touch.
 42. The method ofclaim 41, wherein establishing the reference level comprises using adelayed sample of the touch signal as the reference level.
 43. Themethod of claim 42, wherein the delayed sample of the touch signal isthe previous sample of the touch signal before the triggering event. 44.The method of claim 41, wherein establishing the reference level inresponse to a triggering event comprises storing each touch signalsample as a possible second reference level until the triggering eventis detected.
 45. The method of claim 35, wherein the reference level isheld unchanged after detection of the application of the touch force.46. The method of claim 41, wherein the triggering event is a rise rateof the touch signal exceeding a predetermined rate.
 47. The method ofclaim 41, wherein the triggering event is a magnitude of the touchsignal exceeding a predetermined value.
 48. The method of claim 35wherein establishing the reference level comprises establishing thereference level based on a particular time at which touch locationinformation is obtained.
 49. The method of claim 48, whereinestablishing the reference level comprises: detecting a quiescent touchsignal, the quiescent touch signal representing the force responsivetouch signal during a period in which no touch force is applied to thetouch screen; extrapolating the quiescent touch signal to the particulartime of the touch location measurement; and using a value of theextrapolated quiescent touch signal at the particular time at which thetouch location is determined as the reference level.
 50. The method ofclaim 48, wherein determining the reference level comprises: detecting afirst quiescent touch signal, the first quiescent touch signalrepresenting the force responsive touch signal before the touch isapplied to the touch screen; detecting a second quiescent touch signal,the second quiescent touch signal representing the force responsivetouch signal after the touch is removed from the touch screen;determining an interpolated touch signal between the first and thesecond quiescent touch signals; and using a value of the interpolatedtouch signal at the particular time at which the touch location isdetermined as the reference level.
 51. The method of claim 50, whereinthe second quiescent touch signal is detected if a touch signal sampleis below a predetermined threshold and at least one subsequent touchsignal sample has a value less than the touch signal sample.
 52. Themethod of claim 50, wherein the second quiescent touch signal isdetected if a predetermined number of successive samples remain within apredetermined range of values.
 53. A touch screen system, comprising: atouch surface; a plurality of touch sensors, each of the touch sensorsphysically coupled to the touch surface and producing a force responsivesignal in response to a touch applied to the touch surface; and acontrol system, coupled to the touch sensors and receiving the sensorsignals, the control system configured to acquire a plurality ofreference levels for the force responsive signal, select one or more ofthe plurality of reference levels based on information acquired from theforce responsive signal, and determine the touch location using the oneor more selected reference levels.
 54. The system of claim 53, whereinthe touch sensors comprise capacitive force sensors.
 55. The system ofclaim 53, wherein the touch surface is substantially rectangular withone of the plurality of touch sensors located at each corner of thetouch screen.
 56. The system of claim 53, wherein each touch sensorproduces a sensor signal indicative of a force passing through thelocation of that touch sensor.
 57. The system of claim 53, wherein thecontrol system derives one or more force responsive touch signals bycombining one or more sensor signals.
 58. The system of claim 53,wherein the control system develops a first and a second reference leveland determines the touch location using at least one of the first andthe second reference levels.
 59. A touch screen display system,comprising: a touch screen system, including: a touch surface; aplurality of touch sensors, each of the touch sensors physically coupledto the touch surface and producing a force responsive signal in responseto a touch applied to the touch surface; and a control system, coupledto the touch sensors and receiving the sensor signals, the controlsystem configured to acquire a plurality of reference levels for theforce responsive signal, select one or more of the plurality ofreference levels based on information acquired from the force responsivesignal, and determine the touch location using the one or more selectedreference levels; and a display for displaying information through thetouch screen system.
 60. The system of claim 59, wherein the display isa liquid crystal display, a light emitting diode display, a plasmadisplay, and organic electroluminescent display, or a cathode ray tubedisplay.
 61. The system of claim 59, wherein each touch sensor producesa sensor signal indicative of a force of a touch passing through thattouch sensor.
 62. The system of claim 59, wherein the control systemderives one or more touch signals by combining one or more sensorsignals.
 63. A display system, comprising: a touch screen system,including a touch surface; a plurality of touch sensors, each of thetouch sensors physically coupled to the touch surface and producing aforce responsive signal in response to a touch applied to the touchsurface; and a control system, coupled to the touch sensors andreceiving the sensor signals, the control system configured to acquire aplurality of reference levels for the force responsive signal, selectone or more of the plurality of reference levels based on informationacquired from the force responsive signal, and determine the touchlocation using the selected reference levels; a display for displayinginformation; and a processor coupled to the display and the touch screensystem for processing data to be displayed on the display andinformation received from the touch screen system.
 64. The system ofclaim 63, wherein the display displays information through the touchscreen.
 65. The system of claim 63, wherein the display is a liquidcrystal display, a light emitting diode display, a plasma display, andorganic electroluminescent display, or a cathode ray tube display. 66.The system of claim 63, wherein the processor receives informationregarding a touch made on the touch screen relative to informationdisplayed on the display.
 67. The system of claim 63, wherein each touchsensor produces a sensor signal indicative of a force passing throughthe location of that touch sensor.
 68. The system of claim 63, whereinthe control system derives one or more touch signals by combining one ormore sensor signals.
 69. The system of claim 63, further comprising: oneor more data storage devices coupled to the processor for storing data;one or more input devices for transferring information to the processor;and one or more output devices for transferring information from theprocessor.
 70. The system of claim 63, further comprising one or moreinterfaces for coupling the system to one or more networks.
 71. A systemfor determining a location of a touch on a touch screen, comprising:means for developing a plurality of reference levels for a forceresponsive touch signal; and means for determining the location of thetouch using one or more of the plurality of established referencelevels.
 72. The system of claim 71, wherein means for determining thelocation of the touch comprises means for transitioning from using onereference level as a touch signal reference to using other referencelevels as the touch signal reference.
 73. The system of claim 72,wherein transitioning from using one reference level as the touch signalreference to using other reference levels as the touch signal referencecomprises means for eliminating an effect associated with one or morereference levels from the touch location determination after apredetermined time period.
 74. A system for determining a touch locationon a touch screen, comprising: means for developing a first and a secondreference level for a force responsive touch signal; and means fordetermining the touch location using at least one reference level of thefirst and the second reference levels.
 75. The system of claim 74,wherein means for developing the first reference level comprises meansfor adjusting the first reference level by a predetermined amount when adifference between a magnitude of a touch signal sample and a magnitudeof the first reference level is less than a predetermined value.
 76. Thesystem of claim 74, wherein means for developing the second referencelevel comprises means for establishing a touch signal value acquiredcontemporaneously with the detection of a touch force as the secondreference level.
 77. The system of claim 76, wherein means forestablishing a touch signal value acquired contemporaneously with thedetection of the touch force comprises means for detecting a triggeringevent.
 78. The system of claim 77, wherein means for detecting atriggering event comprises means for detecting a rise rate of the touchsignal exceeding a predetermined rate.
 79. The system of claim 77,wherein means for detecting a triggering event comprises means fordetecting a touch signal magnitude exceeding a predetermined value. 80.A system for establishing a reference level for a touch signal used todetermine a touch location, comprising: means for sensing a quiescenttouch signal prior to an application of a touch force; means fordetecting a touch signal responsive to the application of the touchforce; means for establishing a reference level for the touch signalbased on a value of the touch signal acquired contemporaneously with thedetection of the touch force.
 81. The system of claim 80, wherein meansfor detecting the touch signal responsive to the application of thetouch force comprises means for detecting a triggering event associatedwith the application of the touch force.
 82. The system of claim 81,wherein means for detecting the triggering event comprises means fordetecting a rise rate of the touch signal exceeding a predeterminedrate.
 83. The system of claim 81, wherein means for detecting thetriggering event comprises means for detecting a magnitude of the touchsignal exceeding a predetermined value.
 84. The system of claim 80wherein means for establishing the reference level comprises means forestablishing the reference level based on a particular time at whichtouch location information is obtained.
 85. A computer-readable mediumconfigured with executable instructions for causing one or morecomputers to perform a method of determining a location of a touch on atouch screen, the method comprising: acquiring a plurality of referencelevels for a force responsive touch signal; selecting one or more of theplurality of reference levels based on information acquired from thetouch signal; and determining the touch location using the selectedreference levels.